Add Canonical XML serialization modes

[FlorianRappl]

Description

Add canonicalization output modes to XML serialization to support deterministic, standards-compliant canonical forms.

Proposed support:

• Canonical XML 1.1 profile
• Exclusive XML Canonicalization profile
• clear API for selecting canonicalization mode

This should normalize namespace and attribute ordering according to canonicalization rules.

Background

User scenarios include:

• XML digital signatures
• reproducible build artifacts
• stable diffs and snapshot testing
• cross-system interoperability where canonical form is required

Current serializer behavior is practical for general output but not aimed at canonicalization standards.

Specification

Relevant specifications:

• Canonical XML 1.1
• Exclusive XML Canonicalization Version 1.0
• XML Signature workflows that depend on canonicalized XML input